I tried to reset my macbook air and now a folder with a question mark only appears. How can I fix it?

I'm giving my old macbook air away to my friend and i tried to reset it. I followed instructions online, I went to disk utility and erased macintosh and something else. When it restared a folder with a question mark appeared. My mom thinks I deleted the hard drive and the computer is ruined. How can I fix it, please help!